1. Field of the Invention
The present invention relates to downhole filters, methods of filtering production fluid downhole, and methods of producing downhole filters. Embodiments of the invention relate to downhole filters, such as sand screens, for use in preventing sand or other particulates entrained in production fluid from passing from a producing formation into a wellbore.
2. Description of the Related Art
It is generally desirable that fluids extracted from downhole formations, such as oil and gas produced from hydrocarbon-bearing formations, are substantially free from particulates, or sand. The presence of sand in the production fluid can lead to blockages, premature wear and damage to valves, pumps and the like. Produced sand which has been separated from the produced fluid at surface requires storage and disposal, which can be difficult and expensive, particularly in offshore operations. Furthermore, unchecked production of sand from a formation can result in substantial damage to the formation itself.
Perhaps the most common means for restricting sand production involves the provision of a mechanical sand control device, installed downhole, that causes the sand to bridge or filters the produced liquids or gases. These devices come in many forms, including slotted liners and wire-wrapped screens. The simplest slotted liner is made of oilfield pipe that has been longitudinally slotted with a precision saw or mill. Such liner is relatively inexpensive, and is accordingly preferred for wells having long completion intervals, but does not have high-inlet-flow areas, and may therefore be unsuitable for high-rate wells. Wire-wrapped screens consist of keystone-shaped corrosion-resistant wire wrapped around a drilled or slotted mandrel, the wire being spaced from the mandrel by longitudinal ribs to allow for maximum flow through the screen.
Other sand control devices comprise a filter sheet sandwiched between a perforated base pipe and a perforated outer shroud. By providing the filter sheet in the form of a plurality of overlapping leaves, and providing a diametrically expandable base pipe and outer shroud, it is possible to provide an expandable sand control device, such as is sold under the ESS trade mark by the applicant. In this particular arrangement, overlapping leaves of non-expanding apertured metal filter sheet are sandwiched between a slotted expandable base pipe and a slotted expandable protective shroud. Each leaf is attached to the base pipe along an axially extending weld, and the free edges of the leaves then overlapped to provide an iris-like arrangement. On expansion of the filter, the leaves of filter sheet slide over one another, the circumferential extent of each leaf being selected such that a degree of overlap remains in the expanded configuration, such that there is a continuous wrapping of filter sheet.
While such expandable filter arrangements have been used successfully on many occasions, manufacture of the arrangements is relatively difficult and expensive, and the location and relative movement of the filter sheets during the expansion process introduces a risk of the filter sheets tearing. When installing the sand control device as a completion string within the wellbore, the outer shroud may tear upon coming into contact with an obstruction within the wellbore, rendering the sand control device ineffective for its desired purpose. Installing a filter arrangement downhole is especially problematic when it is desired to drill to the desired depth within the formation using the filter arrangement, as the outer shroud is especially prone to tearing upon portions of the formation while drilling.
Embodiments of the various aspects of the present invention provide alternative sand control devices.